In 1999, he voted to prohibit gay couples in Washington from adopting.

Two years ago, his son told him he was gay.

And this week, Sen. Rob Portman, R-Ohio, announced a change of heart, saying he had decided to support same-sex marriage. The stance puts him at odds with two-thirds of Republican voters and the rest of his fellow Republicans in the Senate.

“I have come to believe that if two people are prepared to make a lifetime commitment to love and care for each other in good times and in bad, the government shouldn’t deny them the opportunity to get married,” Portman wrote in an essay for the Columbus Dispatch.

For a Midwestern conservative who served in the George W. Bush administration and made Mitt Romney’s shortlist for vice president, Portman’s reversal was both striking and of the moment.

Public opinion has been shifting rapidly in favor of legalizing same-sex marriage, and in two weeks, the Supreme Court will hear arguments on the constitutionality of California’s Proposition 8 and the Defense of Marriage Act. Yet just eight years ago, Portman’s own state was ground zero in a national, and largely successful, movement against gay marriage.

The main reason for Portman’s shift? Family — specifically his son Will, a junior at Yale University.

The Portman family makes big decisions together after much contemplation, said J. Dennis Hastert, a former Republican House speaker who has been close to the Portmans since the senator was in the House leadership in the 1990s.

“Rob’s the kind of person who’d give it a lot of thought and a lot of personal time and insight before he makes a decision like that,” Hastert said. ” … It’s certainly going to rile some people in the party, especially to the right, but I think it’s a personal decision and a decision that goes along with Rob’s thinking. I think he’s very concerned that our party is a big-tent party.”

The announcement surprised former congressman Thomas Davis of Virginia, who also served in House leadership with Portman.

“He’s been traditionally very risk-averse on those issues, but he’s very cerebral,” Davis said. “You do not see any Republicans just coming out on this issue. It’s changing, but this is not the safe place for Republicans with ambition. So you’ve got to think this is a sincere move on his part.”

As the news spread through the political world Friday morning, Portman was unreachable in the Tennessee wilderness, where he was on his annual kayaking and mountain-biking trip with Will and his other son, Jed.

“Especially proud of my dad today,” Will tweeted early Friday, linking to the editorial in the Dispatch.

With the Portmans out of pocket, pundits and politicos were left to haggle over what the announcement means for the Republican Party and Portman’s political future — his current Senate term will be ending just as the 2016 presidential race heats up. Leading Republican gay rights groups praised his decision, while social conservatives decried it.

“Sen. Portman is a great friend and ally, and the speaker respects his position, but the speaker continues to believe that marriage is between a man and a woman,” said Michael Steel, spokesman for House Speaker John Boehner, R-Ohio.

Portman’s party remains solidly against legal recognition of same-sex marriages. Seven months ago, the platform approved at the GOP convention in Tampa reiterated the party’s opposition and endorsed a constitutional amendment limiting marriage to one man and one woman.

At this week’s Conservative Political Action Conference, at National Harbor, Texas Gov.Rick Perry, a Republican, decried the “media narrative” that the country is pulling away from conservative ideals. When asked whether Portman’s announcement was a turning point for the party, anti-tax advocate Grover Norquist said it remains to be seen.

“The discussions of exactly how you deal with marriage is an important issue,” Norquist said during the conference. “But the most important thing for Republicans to do is make it clear they’re for everybody and they’ll work with everybody and they want to earn the support of everybody.”

In 2004, Ohio voters backed a state constitutional amendment banning gay marriage by 62 percent to 38 percent. By last fall, a Washington Post poll showed that 52 percent of registered voters in the state said gay marriage should be legal.

On the national level, more than 100 prominent Republicans sent a brief to the Supreme Court in February supporting the legalization of gay marriage. But only 29 percent of Republican voters agree, according to a Washington Post-ABC News poll in November.

Portman, known as a workhorse who is high on thoughtfulness and low on charisma, has rarely been vocal on social issues, though his votes have aligned with his party on gay rights. As recently as last year, he opposed the Employment Non-Discrimination Act, which would prevent employees from being fired because of their sexuality. He has received a failing grade from the Human Rights Campaign, an LGBT rights group.

In an interview with CNN, Portman said he believes that individual states should decide whether to allow gay marriage. He also described how he informed the Romney campaign of his son’s sexuality during the vice-presidential vetting process — Portman added that this wasn’t a factor in Romney’s decision to pick someone else — and how Will endorsed his father’s use of his name and sexuality in the announcement.

“I think [Will is] happy and, you know, proud that we’ve come to this point, but he let it be my decision just as, you know, it’s going to be his decision as to the role he plays going forward in this whole issue,” said Portman.

Portman also said he sought counsel from former vice president Dick Cheney, a supporter of gay marriage whose daughter Mary is married to a woman.